1. Field of the Invention
The present invention relates to a welding device and a carbon dioxide gas shielded arc welding method.
2. Description of the Background Art
Japanese Patent Laying-Open No. 60-108179 (Patent Literature 1) discloses a consumable electrode type arc welding method repeating short circuiting and arcing between the consumable electrode and the base metal. In this consumable electrode arc welding method, the step of forming a droplet and the step of transferring the droplet to the base metal are repeated.
FIG. 9 is a diagram to describe a consumable electrode type arc welding method repeating short circuiting and arcing.
Referring to FIG. 9, in a consumable electrode type arc welding method repeating short circuiting and arcing, the steps of (a)-(f) that will be described hereinafter are sequentially executed repeatedly. (a) A short circuit initial state in which a droplet forms contact with a molten pool; (b) a short circuit middle state in which the contact between the droplet and the molten pool is definite such that the droplet is transferring towards the molten pool; (c) a short circuit latter state in which the droplet is transferred to the molten pool side with a constriction between the weld wire and molten pool; (d) the short circuit is broken and an arc is generated; (e) an arcing state in which the tip of the welding wire melts to form the droplet; and (f) an arcing state in which the droplet grows, immediately before short circuiting with the molten pool.
In the conventional short circuiting transfer welding disclosed in Japanese Patent Laying-Open No. 60-108179, arcing and short circuiting occur on a regular basis. However, according to globular transfer associated with short circuiting in the case where welding is performed by a carbon dioxide gas shielded arc welding method at a high current (>200 A), a droplet will be elevated to the upper portion of the wire by the repelled force by the arc, increasing the arcing time. It will become difficult to cause short circuit periodically, and the arcing and short circuiting will occur irregularly.
Such an irregular change of the short circuit period and arc period will cause an indefinite droplet size at the time of short circuiting, leading to an irregular bead shape.
Furthermore, since a high current will cause excessive repelled force by the arc at a random position relative to the molten pool, the molten pool will be vibrated greatly and irregularly. A humping bead is readily generated by the pushing of the molten pool in a direction opposite to the welding position.
Particularly, there is a demand for increasing the welding speed (>1 m/s) in order to improve the productivity. Degradation in the welding quality affected by the above-described problem will become noticeable in high-speed welding.